Bridge duplex telegraph system



Patented Jan. 21,V 1930 nin srt tartar ori-*len SIDNEY D. WILBURN, F ENG-LEVOOD, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAYH CGMPANY, ACORPORATION OF NEW YORK BRIDGE DUPLEX TELEGRAPH SYSTEM Application filed January 30, 1928. `Serial No. 250,709.`

This invention relates t0 bridge polar duplex telegraph systems and particularly to means for regulating the line current in th said system which means ensures the maximum steady-state received current for a given applied line voltage.

In the operation ot polar duplex telegraph circuits, it is desirable to maintain the line current within predetermined limits, which l0 limits depend upon the type of line and apparatus used. The regulation ot the line current is usually etlected by varying the resistance ofthe circuit. The variable resistance employed for that purpose is generally associated with the duplex set instead ci? with t-he line because, as is Well known, it is frequently desirable to switch a duplex set from one line toanother that may differ in resistance from the former. As will be seen from subsequent description of the invention, the current regulating resistance may be located in the bridge duplex circuit at either oi' three places, namely, (l) a single resistance in series with y F the battery branch `of the circuit or (2) equal' 2D resistances in series with each of the bridge arms or (3) equal resistances in series with the line and the artificial line o' the duplex set.

My invention resides in the placing ot equal Y resistance in series with each bridge arm ot the duplex set, and in so proportioning such resistances (the mannerot doing which is hereinafter set forth) that the maximum steady-state received current may be attained with a given applied line voltage. r

i Other objects of this invention will be apparent from the following description when read in connection with the attached drawing` of which Figure l shows schematically7 a bridge duplex telegraph circuit having the current limiting resistances in the bridge arms of the duplex set; Figs. la and 1b which are portionso a duplex circuit andlshow other possible positions torthe current limiting resistance or resistances are, together with Figs. 2 and 3, intended to illustrate the description of the invention.

In Fig. l, the bridge arms at station X comprise the windings l() and 1l and the variable resistances 12 and 13. In subsequent equations the value of 10 and l2 is represented by R2 and the value 01"' ll and 13 is also represented by R2. The duplex set is connected with the line and the artificial line as shown in the figure and has a receiving re lay connected across the bridge arms. The bridge arms o1 the duplex set connected with the other end ot the line at station Y are similar to those just described. rlhe circuit needs no further description because it is ot a type well known in the art.A The resistances l2 and 13 are in the form of a double rheostat with movable arms mechanically connected to facilitate adding equal amounts of resistance simultaneously to both arms. It is to be understood that, although the elements 10 and l1 (and their primes) have been shown as inductances, such elements are not so limited inasmuch as resistances may be substituted tor the said inductances. v

In Fig. la, the current regulating resistance r which is indicated by the symbol Rl is `located in the battery branch of the circuit.

In Fig. 1b, the current regulating resistances'indicated by the symbol Its are connected between the line and the artificial line and the duplex set, the said resistancesbeing in the form of a double rheostat as in Fig. l.

As will be subsequently shown, the arrangement in Fig. l produces the maximum steadystate received current for a given applied line voltage and is also most satisfactory from a transmission standpoint inthat it does not i adversely aiiect the shape of the received cur- AThat the arrangement `shown in Fig. l is superior to those shown in Figs. laL and l", and also to other known arrangements, will be apparent from consideration of the following equations representing the relationships that exist between parts of the arrangements shown in Figs. 1 la and lb.

In setting up t-he equations tor the received current and the line current, certain prac- T, the resistance of the line between the duplex sets. R1, R2 and R3 are the regulating resistances in the different locations corresponding to the subscripts. In the equations for arrangement (l), G is assumed to be contained in Rl and in the equations ttor arrangement (2), a is assumed to be contained in R2. The equations for the parameters, r1, r2 and r3 are as follows:

and the expressions for the ratios of the received currents to corresponding line cur- 5 rents, I1, I2 and I3 are:

a, represents the constant resist-ance of each bridge arm in arrangements (l) and g I), the resistance of the receiving relay; E, the voltage of the line battery which is il assumed to be equal at both stations and may be either negative or positive;

G, the constant resistance in series with the line battery taps in arrangements (2) and `While the line current and the received current can be calculated for any values of R and T from Equations (l) to (9) inclusive, explicit line current equations are needed for calculating the received current for a definite value of line current, such as shown by curves m1, wg and ma, Fig. 2. It is clear that these curves cannot be calculated from Equations (l) and (9) alone, as the first step necessary is to determine the value of R which, With a given value of T, will result in the speciied value of I (.070 ampere). IVith line current equations, R can, of course, be calculated by substituting .070 for I. While the line current equations can be set up fairly readily, they are of an extremely cumbersome character. F or that reason curves m1, an and wn, Fig. 2, were calculated by the following method:

From Equations (l) to (9) inclusive, the line current was calculated for the various values of T from 500 to 3000 ohms with various values of R from O to 2000 ohms in steps of 250 ohms. For each value of T the line current Was then plotted against R and the required value of the latter read from the intersection of the curve and the .070 ordinate. The values of R thus obtained were then subsituted in Equations (7) to (9) for calculating r. turn Were substituted in Equations (l) to By the above method of values of R Within plus or minus two or three ohms can be determined. This possible error in R Will not appreciably allect the points on the curves. The point of intersection of m1, m2, w3 and x0, F ig. f2 was calculated by equating the right hand of Equation to .0138.

Referring to Equations (l), (2) and (3) showing the relations between the regulating resistances and the received currents, it Will be noted that in the right hand member of 1) and (3) R-1 and R3, respectively, appear only as positive terms in the denominator. This shows that the received current will inevita- These values of R and r in bly be reduced for every increase in the resistance, provided r1 and r3 are continuously increasing functions of R1 and R3 and from Equations (7) and it will be seen that both r1 and r3 increase continuously for every increase in R1 and R3, respectively. In Equation (2), however, R2 appears in both the numerator and the denominator and in the latter it appears in both the first and e `10 second powers.

`does not lshow just how the received current will be affected by increasing the resistances, the difference `in the equation for the received current offers a guide in the selection of the location for the resistance which results in the greatest received current. Closer inspection of Equation shows that when R2 equals zero, the received current will be Zero and since the denominator of the right hand member contains the second power of R2, the received currentwill approach zero if R2 be increased indeinitely. Furthermore, there will be current in the receiving relay for all finite values of R2. The nature of the equation shows also that 2 `will have but one maximum. Tf the value of R2 corresponding to maximum 712 proves to' be greater than 500 ohms, it will open upthe possibility ofincreasing the received current by adding the regulating resistances to the bridge arms as shown in Fig. 1. e

e In order to determine the value of R2 corresponding tothe maximum 2, it is necessary to differentiate Equation (2) with respect to R2 and `toecmate it to zero. The expression to be dealt with in this differentiation is that which results from the substitution of the right hand member of Equation (8) for r2 in Equation (2). This substitution gives the following equation for the steady-state current in receiving relays of abalanced symnietrical bridge duplex telegraph circuit.

resistance and a battery tap resistance of 120 ohms, Equation (11) becomes R2= 10a/3 (T+ arl/2+ 320) From this equation, it is found that the ly 555 ohms. rWith line circuits ranging in resistance from 1000 to 2500 ohms, the respective values of R2 necessary for maximum received current strength, range from approximately 624 to 935 ohms. If, then, resistance be added in the proper amounts by the devices 12 and 13, Fig. 1, the received current will be `increased thereby and at the same time, the line current will be reduced. If the line circuit resistance is approximately 1650 ohmsor more the amount of resistance needed from 12 and 13 to make the received current maximum will be sufiicient to reduce the line current to .07 0 ampere or less. This is illustrated by the three upper curves in. Fig. 2. The lower broken curve, designated 000 represents the received current which will be obtained with no regulating resistance in the circuit at either point. It will be seen that this curve passes below curve 002 at a point corresponding to a line resistance of 1650 ohms. lith approximately that value of line resistance and no regulating resistance, the line current is approximately .086 ampere and the received current is .0171 ampere. If approximately 4104ohms be added by 12 and 13 the line currentwill be reduced to .070 ampere and the received current will first rise to about .0174: ampere as the first resistance is added and then recede to its original pointof .0171 ampere. Thus, the line current will have been reduced without reducing the received current. On vthe other hand, had the regulating resistance been addy Since the diderentiations of Equation (10) results in an equation involving powers of R2 upto the sixth together with an unusual large number of terms, it is impractical to use this.

It has been found, however, that forvalues of4 R2 andT within the practical ranges of 600 to 1750 ohms for R2 and 500 t@ 3000 @gms for T, r2 is nearly equal to 1/3122 +21/T+ 200.

If this expression be substituted for r2 in `Equation (2) and this result diiferentiated and equated to zero, it leads to the following equation which gives values of R2 approximating closely the point of maximum received current. This equation is:

R2= vs /"Lib (11+ ZTI/w+ 200 11), Assuming thereceiving relay of L100 ohms current would have been reduced to .014:7 and .0138 ampere, respectively. Curve mum, shows themaximum received current which can be realized by adding correct amounts of resistance to `the bridge arms. The upper curvetouches curve m2 at a point corresponding to a line resistance of 1850 ohms. That is, with a line resistance of this value, the regulating resistance requiredto reduce the line current to .070 ampere is just suiiicient to bring the received current up to the maximum. For lines of this resistance or greater, the line current can be reduced to .070 ampere or less and at the same time the received current is increased.` lt will be seen from Fig. 2, that as compared to locations shown `in Figs;

'ies

lio

ias

idc

1a and 1b for the regulating resistance, the advantage et location in Fig. 1 from a steadystate received current standpoint, becomes greater with lines of low resistance and amounts to 32.3% and 60.1% respectively, with a line of 500 ohms resistance. On the other hand, the increase in received current due to arrangement in Fig. 1, as compared to the condition of ne regulating resistance, becomes greater with lines of higher resistance, as shown by the divergence of the m2 and 'mmm curves, Fig. 2.

Fig. 3 represents Equation (10) plotted for a 1200 ohm line and it will be seen that it is better to have the bridge arm too great in magnitude rather than too small, since the received current rises rapidly to a maximum and then descends slowly. On the other hand, if resistances be added at the points shown in Figs. 1a and 1, the operating point on the received current curve will, in all cases, be moved further away from the maximum and this movement will take place on the side of the maxium which has the greatest affect in reducing the received current.

It will be apparent that from the foregoing disclosure of the relationship existing between the received current and the line current and the regulating resistance under the situations represented by Figs. 1, 1a and 1b, that the employment of the current regulating resistances in the bridge arms of the dupleX set results in the attainment of the maximum received current for any given value of applied line voltage. Furthermore, as briefiy referred to hereinbefore, the oscillographic tests made upon the circuit shown in Fig. 1 indicates that the arrangement therein employed improves the wave shape of the received current as compared with other possible arrangements considered.

Vhile the invention has been shown as embodied in a particular form, it is capable of embodiment in other and different forms without departing from the spirit and scope of the appended claims.

lVhat is claimed is:

1. In a bridge polar duplex telegraph systern, the combination with a line of a bridge polar duplex terminal circuit connected with each end thereof, each terminal circuit including a polar receiving relay, an artificial line, two ratioarms, and a line battery tap containing a resistance connectedwith the apex of the said arms, the said resistance in each ratio arm being in value equal to where b is the resistance of the receiving relay, T is the resistance of the line between duplex sets, and G is the constant resistance each end thereof', each circuit comprising ratio arms, an artiiicial line, a polar receiving relay connected across said ratio arms between the line and the artificial line and a battery tap connected with the apex of the ratio arms, the said ratio arms, each comprising the same amount of resistance, which amount equals 1/3/46 (T+ 2T1l2+ G+ 200),

where 7) equals the resistance of the receiving relay, T is the resistance of the line between duplex sets, and G is the constant resistance in series with the line battery tap.

3. The method of controlling` the magnitude of the line current in a bridge polar duplex telegraph system comprising a line having a bridge polar duplex terminal circuit connected with each end thereof, each terminal circuit including a polar receiving relay, an artificial line, two ratio arms, and a line battery tap containing a resistance connected with the apex of the said arms, which method consists in introducing an equal amount of resistance in each ratio arm of such magnitude that the resistance of the ratio arm shall be equal to where b is the resistance of the receiving relay, T is the resistance of the line between duplex sets, and G is the constant resistance in series with the line battery tap.

4. The method of controlling the magnitude of the line current and of improving the wave shape in a bridge polar duplex telegraph system which consists in introducing in the ratio arms of the bridge equal amounts oi' resistance of such value as to give maximum received current for a given applied line voltage.

In testimony whereof, I have signed my name to this specification this 28th day of January, 1928.

SIDNEY D. WILBURN 

